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I want to show you a reaction which I
didn't know could occur the reaction
between magnesium and water the reason
why I didn't know it can occur is quite
simple let's look at this experiment
here I've got a piece of magnesium
ribbon and and some water you see that
if I add the
water absolutely nothing happens I
didn't really think whether magnesium
could react with water however Neil has
done some quite exciting
experiments which Brady has videoed in
slow motion so what Neil did was to take
bits of
magnesium set fire to them in air so
they were burning really brightly and
and then gently lower them into a large
container of water as with all his
experiments he began on quite a small
scale in the first experiment it's all
quite dark you can't see the water very
well at all but you can see the
Magnesium as it comes in
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what I found really quite surprising is
that the burning magnesium continues to
burn
underwater in fact bits of it start
coming out of the water again breaking
off and that came to me as a real
surprise to begin with but then I
realized that the
Magnesium is pretty hot close to its
melting point its melting point is only
650° centigrade and in fact in some of
the shots you can see bits dropping off
the Magnesium even before it gets to the
water because it's
melting when it goes into the water you
have a very clean surface of
magnesium and so there is no oxide layer
to protect the Magnesium as there is in
the water that we've got here where the
Magnesium is coated in a layer of
oxide so it's insulated if you like from
the water so you have very high
temperature water and clean
magnesium and so you can get a reaction
of magnesium producing magnesium oxide
and hydrogen it may be magnesium
hydroxide but you will get
hydrogen so you have an exothermic
reaction making the magnesium oxide or
hydroxide and you're producing hydrogen
which itself can burn and could cause a
minor explosion it's a bit like sodium
but not quite so
violent Neil then tried it on a bigger
scale and there it's really nice you can
see as the Magnesium goes close to the
water surface you can see Its Reflection
coming up to meet it the reflection of
the burning
magnesium and
then there is a few seconds of video
which was a much shorter time because
it's been slowed down in
reality then there's really quite a
violent reaction and pieces of burning
magnesium fly out in fact some fly out
completely outside the
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container then being
Neil he tried an even bigger piece
then what happened is that again you see
the burning
magnesium approaching Its Reflection and
once it goes in you get a really violent
reaction there is quite a nice red color
that is formed
I'm not
sure whether this is the emission from
hot magnesium atoms we've seen these
sort of emissions in the reactions of
Rubidium and sodium or whether it is
just the hydrogen gas burning because
we've also seen in some of videos that
hydrogen gas in the absence of excess
air will burn with a reddish color but
there is a quite a nice reddish color
formed as well so what can we learn from
all of these chemistry is often
unexpected it I'd never seen the
reaction of magnesium with water and it
is the high temperature and the fact
that it's liquid that is causing this
reaction to take place if you look at
the periodic table calcium reacts quite
easily with water calcium is the element
immediately below magnesium so it's not
surprising that magnesium also reacts
with water but chemists usually forget
that I did ask Neil
why he' ever tried this
experiment and the answer is that he was
burning a piece of magnesium in his fume
coverbed and the fire got a bit bigger
than he really liked so he put it into
some water and his amazement it went on
burning so the real take-home message
for all of you is that if you have a
metal fire the last thing you want to do
is to put water on it this is probably
why during the world wars people used
magnesium bombs as incend devices
because once the bomb starts burning you
can't put it out with water Sometimes
they used phosphorus often they used
magnesium if ever you're working with
Metals at high temperature and they
start burning use sand don't use water
Professor if we scraped the oxide off
that piece of magnesium ribbon would it
react with the water at all like do you
need the
heat I have no idea I wondered myself
whether this would happen my feeling is
that there might be a reaction but it
would be pretty slow that is if we clean
the Magnesium perhaps electrochemically
or something like that eventually we
might see a few bubbles of hydrogen but
it's partly the high temperature the
molten metal that really gets the thing
to take off so the answer is it might
react but it wouldn't make a good
video so if you'd like to see another
great magnesium reaction you can see the
video we made a little while ago of
magnesium reacting with solid CO2 again
magnesium is extracting oxygen from a
molecule CO2 that we would normally
expect to be pretty stable the
difference between its reaction with
water and CO2 is that the carbon it
leaves doesn't burn you just see a black
residue the hydrogen adds a little WP to
the experiment